1. Field of the Invention
This invention relates to an organic electroluminescent element and especially to such an element having improved film-forming properties and a production process thereof.
2. Related Arts
Recently, in accordance with diversification of information apparatuses, the demand has been growing for flat-type display elements which consume less electric power and need smaller space than cathode-ray tubes (CRT). There are a crystalline liquid device, a plasma display device and the like as such a flat-type display device. Now, an electroluminescent (hereinafter referred to as EL) element which is a self-luminescent type and provides clear display has been drawing special attention.
Here, the EL elements can be divided between inorganic EL elements and organic EL elements depending on the constituent materials, the former having already been put to practical use.
However, the inorganic EL element needs to be driven with high voltage because its driving type is so called `Collisional excitation type` wherein accelerated electrons impressed in a high electrical field make the luminescence center luminesce through collisional excitation. This leads to an increase in the cost of surrounding devices. On the other hand, the organic EL element can be driven with low voltage because it is a charge (hole and electron) injected from an electrode. It has another advantage of being able to gain any luminous colors easily by modifying molecular structure of the organic compounds. Therefore, the organic EL element is very promising as a new display element.
The organic EL element is generally formed with either a two-layer structure or a three-layer structure. The two-layer structure has either a SH-A structure wherein a hole transport layer and a luminous layer are formed between a hole injection electrode and an electron injection electrode, or a SH-B structure wherein a luminous layer and an electron transport layer are formed between a hole injection electrode and an electron injection electrode. The three-layer structure has a DH structure wherein a hole transport layer, a luminous layer and an electron transport layer are formed between a hole injection electrode and an electron injection electrode. Electrode materials having a large work function such as gold and ITO (In--Sn oxide) are used as the above hole injection electrodes, while electrode materials having a small work function such as Mg are used as the above electron injection electrodes. Organic materials are used for the above hole transport layers, luminous layers and electron transport layers. Materials having P-type semi-conductor characteristics are used for the hole transport layers, and those having N-type semi-conductor characteristics are used for the electron transport layers. Materials having N-type semi-conductor characteristics are used in the SH-A structure, those having P-type semi-conductor characteristics are used in the SH-B structure and those having characteristics close to neutral are used in the DH structure for the above luminous layers. Every structure described above is based on a common principle that holes injected from a hole injection electrode and electrons injected from an electron injection electrode recombine on the boundary surface of a luminous layer and a hole (or electron) transport layer, causing the inside of the luminous layer to luminesce.
The choice of materials in the above organic element greatly affects its various characteristics such as luminous efficiency and durability.
For example, 1,1,4,4,-tetraphenyl-1, 3-butadiene derivatives and styrylbenzene derivatives are proposed as blue luminescent materials in a luminous layer, but their film-forming properties are too poor to gain sufficient luminance and stability.
Also, tBu-PBD [2-(4'-tert-Butylphenyl)-5-(4"-biphenyl)-1,3,4-oxadiazole], perylene derivatives and the like are known as electron transport materials. The former makes the durability of EL elements deteriorate because of its poor film-forming properties, while the latter has a restricted use as an electron transport layer because its fluorescent wavelength is around 600-800 nm. This means that when the fluorescent wavelength of an electron transport layer is longer than that of a luminous layer, excitons formed in the luminous layer move to the electron transport layer to disappear.
Also, several kinds are known as hole transport materials and typical among them are a compound with an oxadiazole ring inside the molecular structure, and a diamine derivative. These compounds have excellent hole transport characteristics and have been used as organic EL materials but they have a problem of being destroyed after film-forming due to their being easily crystallized.